Does anyone know how to replace nulls in a column with a string until it hits a new string then that string replaces all null values below it? I have a column that looks like this
Original Column:
PAST_DUE_COL 91 or more days pastdue Null Null 61-90 days past due Null Null 31-60 days past due Null 0-30 days past due Null Null Null
Expected Result Column:
PAST_DUE_COL 91 or more days past due 91 or more days past due 91 or more days past due 61-90 days past due 61-90 days past due 61-90 days past due 31-60 days past due 31-60 days past due 0-30 days past due 0-30 days past due 0-30 days past due 0-30 days past due
Essentially I want the first string in the column to replace all null values below it until the next string. Then that string will replace all nulls below it until the next string and so on.
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Answer
SQL Server does not support the ignore nulls option for window functions such as lead() and lag(), for which this question was a nice fit.
We can work around this with some gaps and island technique:
select
t.*,
max(past_due_col) over(partition by grp) new_past_due_col
from (
select
t.*,
sum(case when past_due_col is null then 0 else 1 end)
over(order by id) grp
from mytable t
) t
The subquery does a window sum that increments everytime a non null value is found: this defines groups of rows that contain a non-null value followed by null values.
Then, the outer uses a window max() to retrieve the (only) non-null value in each group.
This assumes that a column can be used to order the records (I called it id).
ID | PAST_DUE_COL | grp | new_past_due_col -: | :---------------------- | --: | :---------------------- 1 | 91 or more days pastdue | 1 | 91 or more days pastdue 2 | null | 1 | 91 or more days pastdue 3 | null | 1 | 91 or more days pastdue 4 | 61-90 days past due | 2 | 61-90 days past due 5 | null | 2 | 61-90 days past due 6 | null | 2 | 61-90 days past due 7 | 31-60 days past due | 3 | 31-60 days past due 8 | null | 3 | 31-60 days past due 9 | 0-30 days past due | 4 | 0-30 days past due 10 | null | 4 | 0-30 days past due 11 | null | 4 | 0-30 days past due 12 | null | 4 | 0-30 days past due